1. Field of the Invention
The present invention relates to a conductive interconnected porous film and a method of production of the same, more particularly relates to a conductive interconnected porous film which secures conductivity in a film of a porous resin and a method of production of that conductive interconnected porous film.
2. Description of the Related Art
In a general solid polymer electrolyte fuel cell (PEFC), separators are arranged at both of a fuel electrode which supplies hydrogen (anode) and an air electrode which supplies oxygen (cathode). The two separators grip a membrane electrode assembly between them. This membrane electrode assembly is comprised of a proton conductive film on the two sides of which catalyst layers, water-repelling layers, and further, at the outsides of those, gas diffusion layers are superposed. The gas diffusion layers cause the gas which is supplied from the separator sides to the catalyst layer sides to diffuse to make the concentrations uniform. Therefore, the gas concentration at the membrane electrode assembly becomes constant at all times, uneven reaction is eliminated, and the power generating efficiency rises.
A first property which is demanded from such a gas diffusion layer is that transport of electrons between the separators and catalyst layers not be inhibited, that is, that the conductivity be high. A second is the provision of a good gas permeability convenient for feed of hydrogen and oxygen from the gas channels of the separators to the catalyst layers and, furthermore, discharge of the reaction product of water (steam). A third is surface flatness to prevent layers from unintentionally contacting and causing short-circuits. A fourth is corrosion resistance and electrical stability for sustained battery performance and longer years of use. A fifth is free of impurities as possible for stabilization of performance.
As the material of the gas diffusion layers provided with the above features, porous metal sheets etc. have been proposed, but these are inferior in corrosion resistance. Therefore, a sheet material obtained by mixing a carbon fiber and binder to form a random net structure has been proposed (see WO01/022509, WO01/056103A, Japanese Unexamined Patent Publication No. 2004-288489A, and Japanese Unexamined Patent Publication No. 2006-143478A, etc.) The materials of the gas diffusion layers which are disclosed in these patent literature use carbon fiber, so are excellent in conductivity and corrosion resistance and low in impurities. Further, since the production results in a net structure, the gas permeability is also excellent. From this, sheet materials made by carbon fiber are the mainstream as the materials for gas diffusion layers which are used for the current day solid polymer type fuel cells.
However, the materials made by carbon fiber which are disclosed in the above patent literature are kept down in amounts of addition of binder so as to maintain a high conductivity. Therefore, when formed to 0.1 mm or less, the sheet materials are poor in resistance to shear force. When formed to 0.3 mm or more, the sheet materials are poor in flexibility and difficult to roll up etc. Further, even sheet materials which are formed to 0.1 to 0.3 mm easily crack when bent. In addition, the sheet materials easily shed carbon fibers. In this way, sheet materials made from carbon fiber used as the existing materials of gas diffusion layers are fragile, so are not necessarily high in convenience in handling at the time of production.
Therefore, a new porous material which is provided with conductivity which satisfies the requirements of a good conductivity, good gas permeability, surface smoothness, corrosion resistance, and low impurities and which is strong in bending and excellent in handling has been sought.
As related art, see WO01/022509, WO01/056103A, Japanese Unexamined Patent Publication No. 2004-288489A, and Japanese Unexamined Patent Publication No. 2006-143478A, etc.